1. Field of the Invention
This invention relates in general to laser printers using multi-faceted rotating polygon deflectors to expose photosensitive media and in particular to laser printers having the capability to print more than one image size and resolution.
2. Description of the Prior Art
In creating digital images on photosensitive media there are at least two different imaging applications with different requirements. The first application is for large format high quality images with fine text and graphics in which the production speed is not critical. These images are up to approximately 12 inches in width and use spot densities over 400 spots per inch. The second application is for smaller format images with large text and a requirement for high production speeds. In these images, the width is about 5 inches or smaller and the spot density is below about 400 spots per inch. In order to minimize the floor space or foot print required by two machines, it would be advantageous to use a single printer in which the format and spot density can be switched easily between the two applications described. In addition, the printer should use a single paper path for both formats and the cost to manufacture such a printer should be lower than that of two separate machines.
The use of laser printers, especially for creating electrostatic images such as in a photocopier, is widely known. In these applications, a single format and single resolution normally are used. High quality laser printers commonly incorporate optical correction means for a defect in the manufacture of polygons called pyramid error, which is a tilt of the polygon facets in such a way that the facets do not form a vertical cylinder. In other words, the facet normals do not all lie in the same plane, but can vary from facet to facet. Even small errors of this type causes extensive degradation of images because the error is periodic, that is, it recurs with each rotation of the polygon. This can lead to a defect in the image called banding. The human eye is very sensitive to banding errors and it would be desirable to eliminate this type of error.
One of the correcting means for polygon pyramid error is to make the facet conjugate to the image forming medium. In U.S. Pat. No. 4,040,096 by Starkweather, this conjugation is provided by a cylindrical lens which has power only in the page scan section of the beam U.S. Pat. No. 4,247,160 by Brueggemann, discloses a laser polygon printer which has a positive cylindrical mirror disposed between a polygon and an image forming medium. However, neither of these inventions prints more than one image format.
Printing more than one image resolution with one printer has been done by using a scanner with a reflecting surface which varies in width in the scanning direction. See G. Starkweather, U.S. Pat. No. 3,944,323. Other methods of using more than one image resolution with one printer are shown in U.S. Pat. Nos. 5,289,001; 5,274,492; 5,255,115; 5,239,313; 4,953,036; 4,734,715; 4,578,689. None of these patents disclose the use of multiple polygons and none use more than one scan length.
The use of more than one format is described in U.S. Pat. Nos. 4,651,170 and 4,651,169 by J. Chandler, D. Kessler, and E. Muka. In these patents, the format in the line scan direction, the scan length, is changed by adjusting the space between the polygon deflector and the image forming medium In order to maintain the beam focus on the medium in the scan section of the beam, the optics prior to the polygon deflector are modified. Also, the cylindrical mirror following the polygon is tilted and shifted in order to maintain the polygon mirror facet conjugate with the image forming medium The page scan format is changed via a speed change in the moving medium.